Field of the Invention
The invention relates to a method for detecting combustion misfires in internal combustion engines, in which a crankshaft speed is evaluated and an influence of stretches of bad road is taken into account.
Combustion misfires in internal combustion engines that have an exhaust gas catalytic converter downstream of the engine can cause damage to the catalytic converter, because high temperatures arise from after-reactions of the uncombusted fuel-air mixture in the catalytic converter. Moreover, combustion misfires also worsen the quality of the exhaust gas.
In order to detect a combustion misfire, a method that measures the instantaneous angular speed of the crankshaft is employed in the present invention.
Such a method is known, for instance, from Published European Application No. 92 111 078.9, corresponding to U.S. application Ser. No. 08/085,351, filed Jun. 30, 1993, which includes the inventors of the instant application. A combustion misfire causes temporary slowing down of the angular speed, which is detected by measuring the periods of time (segment times) during which the crankshaft rotates about a defined angular span.
Those measured times are not used directly. Instead, so-called non-concentricity values are calculated from them, in which various dynamic factors that arise during engine operation (acceleration, deceleration) are compensated for, depending on the method being used. These non-concentricity values are then compared with limit values in order to detect combustion misfires.
In methods in which the non-concentricity caused by the combustion misfire is evaluated, the danger of misdetection exists when the vehicle is traveling over stretches of bad road. The crankshaft may be set into vibration through the drive train. The resultant angular accelerations of the crankshaft can even exceed the values that occur in a combustion misfire. That can cause misdetection.
Such instances of misdetection can be prevented by blanking out the combustion misfire detection when a stretch of bad road appears.
In previously known methods for detecting stretches of bad road, the acceleration of the vehicle body is checked. However, that requires an expensive acceleration sensor.
In order to detect stretches of bad road, the wheel rpm can also be monitored for acceleration values that are higher than the accelerations which are possible with the engine in a particular vehicle. Such high acceleration values then lead to the conclusion of a poor stretch of road.
However, such a method is commercially useful only where such wheel rpm sensors are already present, or in other words in vehicles with an ABS system.